ProjectSummary von Willebrand factor (VWF) is a multimeric glycoprotein in plasma that plays an important role in hemostasis by mediating platelet binding to sites of vascular injury. In recent studies, VWFhasalsobeenimplicatedinmicrovasculardysfunctionandocclusion,inpartbecauseofits unique ability to self?associate and form hyperadhesive strands of enormous sizes attached to the endothelial surface in response to hydrodynamic forces, including shear stress and elongation flow. When these hyperadhesive strands are not removed by the metalloprotease ADAMTS13 in plasma, they bind platelets efficiently, and the accumulation of VWF?platelet thrombi leads to vessel occlusion, tissue infarction, and organ dysfunction. We recently discovered that high density lipoprotein (HDL), a well?known cardioprotective lipoprotein in plasma,anditsmajorcomponentproteinapolipoprotein(Apo)A?I,canattenuatetheextentof VWF self?association, and ultimately the severity of thrombotic complications in the vasculature.ThesestudiesunveiledanovelantithromboticpropertyofHDL/ApoA?I,whichwe hypothesize is very important in diseases characterized by microvascular occlusion. In this application,wewillfocusonthemechanismofVWFself?association,howVWFinteractswith HDL/ApoA?I, and the physiologic impact of the HDL?VWF interaction. In Specific Aim 1, we will identify the VWF self?association site exposed by hydrodynamic forces by use of VWF variants and peptide mapping. In Specific Aim 2, we will determine the effect of VWF self? associationonADAMTS13?mediatedcleavageundershearstress,andassesstheroleofHDLin ADAMTS13?mediated cleavage of VWF in tubes and devices with newly?developed non? adsorptive coatings. In Specific Aim 3, we will evaluate in mouse models of thrombotic microangiopathyandthromboticthrombocytopenicpurpurawhethertheoutcomeanddisease parameters are worsened by HDL or ADAMTS13 deficiency, or improved by HDL or ADAMTS13 treatment. Successful completion of these aims will provide an improved understanding of the basic mechanism of VWF self?association, how this process can be regulated,andhowthisinformationcanbeusedtodevelopnewapproachestotreatthrombotic diseasescausedbydysregulationofVWFself?association.